This invention relates to a fuel supply control method for internal combustion engines after starting, and more particularly to a method of this kind which is intended to set a proper fuel increment applied to a fuel quantity supplied to an internal combustion engine immediately after being restarted in a hot state.
Conventionally, a fuel supply control method for internal combustion engines has generally been employed, which obtains a signal for commanding initiation of air-fuel ratio feedback control based upon an output from an oxygen concentration sensor provided in an exhaust system of the engine, and upon obtainment of the signal, initiates the feedback control of the air-fuel ratio, e.g. the fuel quantity injected by fuel injection valves provided in an intake system of the engine. It has also generally been employed to increase the fuel injection quantity in response to the temperature of an internal combustion engine after starting of the engine and until the air-fuel ratio feedback control is initiated. Amongst these conventional methods, a first method has been proposed by the assignee of the present application by Japanese Provisional Patent Publication (Kokai) No. 61-234237, in which a predetermined lower limit value is provided for the initial value of a fuel increment applied to the fuel injection quantity after starting of an internal combusiton engine in order to secure a certain amount of fuel increase in the fuel injection quantity when the engine is restarted in a hot state. Further, a second method has been proposed also by the assignee of the present application by Japanese Provisional Patent Publication (Kokai) No. 57-70932, in which a time period is counted after starting of an internal combustion engine and until the temperature of the engine reaches a predetermined value, a determination is made as to whether a predetermined time period has elapsed from the time the internal resistance of an oxygen concentration sensor decreased to a predetermined value corresponding to completion of activation of the oxygen concentration sensor, and upon counting up the two kinds of time periods, a signal is obtained for commanding initiation of the air-fuel ratio feedback control.
However, when an internal combustion engine is restarted in a hot state, such as in the event that the engine is once stopped after running at high speeds and started soon again, it often happens that due to high temperature prevailing inside the fuel supply system of the engine, e.g. fuel injection valves gas bubbles are formed in the fuel supply system. This causes a substantial decrease in the fuel quantity supplied to the engine so that the air-fuel mixture supplied to the engine is in effect leaned, which spoils the startability of the engine. This disadvantage cannot be fully overcome by the above-mentioned first method wherein the lower limit value is provided for the initial value of the fuel increment, because a specially large value of the fuel increment has to be set and stored beforehands so as to be applied at restarting of the engine in a hot state, which, however, cannot achieve the air-fuel ratio control in a precise manner. The second method, referred to above, wherein a feedback control initiating signal is obtained upon the lapse of a predetermined time period after completion of the oxygen concentration sensor, cannot fully overcome the above disadvantage, because if after restarting of the hot engine the air-fuel ratio feedback control is initiated immediately upon the lapse of the predetermined time period, which is set at a fixed value irrespective of the engine temperature, gas bubbles contained in the fuel cannot be completely removed, still involving a problem of uncertain air-fuel ratio of the air-fuel mixture supplied to the engine and hence uncertain startability of the engine.